HEMP Workshop

Workshop: High-efficiency materials for photovoltaics

Next generation photovoltaic materials must harness solar energy in new ways if they are to access a fundamentally higher efficiency regime. This workshop brings together specialists in theory and simulation of materials; growth and characterisation of nanostructures; and device physics of solar cells to explore options for new high-efficiency photovoltaics.

HEMP 2015 is over!

The 5th edition of the workshop on High Efficiency Materials for Photovoltaics (HEMP 2015) took place at Imperial College London on the 10th-11th of September, organised by the Quantum Photovoltaics Group (QPV) and the COST Action MultiscaleSolar, and with the support of the Institute of Physics. On this occasion, the workshop was focused on highly mismatched alloys applied to PV (dilute nitrides, bismides and antimonides) but there was also scope for materials for intermediate band solar cells or hot carrier solar cells, among others.

The first session was opened by Andrew Johnson from IQE plc, who focus on the industrial perspective of the future of high efficiency photovoltaics and the role that highly mismatched alloys will play on it. With the context set, Tomos Thomas, from the QPV group, introduced the latest experimental results concerning the GaAsSbN 1 eV subcell as the key building block for all-lattice-matched 4-junction (or more) solar cells. Further applications of bismides in photonic devices were addressed by Prof. Stephen Sweeney, from the University of Surrey, building upon the experience on lasers to discuss their impact on photovoltaics.

The workshop gave a fundamental twist with an invited talk by Prof. Eoin O’Reilly, from the Tyndall Institute of Technology, on the complexities of the bismides band structure and how such theoretical calculations can be exploited to design suitable alloys valuable for PV. The following talks addressed the challenges in the fabrication of highly mismatched alloys: Prof. Mircea Guina, from the Tampere University of Technology, spoke on GaInNAsSb alloys – with absorption edges ranging from 0.8 eV to 1.4 eV when lattice matched to GaAs, and Prof. John David, from the University of Sheffield spoke on the growth of GaAsBi alloys for photodiodes and solar cells. Finally, Louise Hirst, from the Naval Research Laboratory, focused on the growth of InAlAsSb alloys lattice matched to InP, another material system but that share some of the complexities of nitrides and bismides, such as miscibility gap and the need of post-growth annealing to have good properties.

Highly mismatched alloys and their unique band structure can also be used to exploit intermediate band effects. Prof. Stanko Tomic, from the University of Salford, centred on the use of dilute nitrides in nanostructures, both quantum wells and dots. In turn, Nair Lopez, from the Universidad Autonoma de Madrid, gave an invited talk on the use of bulk dilute nitrides as intermediate band solar cells, showing experimental proof of the working principles of the concept with these type of alloys.

The final presentation involving highly mismatched alloys was the invited talk by Prof. Vesselin Donchev, from the University of Sofia, on the surface photovoltage spectroscopy as a versatile technique to characterise the properties of bulk materials, including dilute nitrides, or nanostructures.

The potential of nanostructures for PV, no longer limited to nitrides or bismides, was addressed first by Prof. Takashi Kita, from the Kobe University, on the dynamics of two photon absorption processes necessary for intermediate band operation by implementing a photon ratchet as a coupled multiple quantum well superlattice. Sabina Hatch, from the University College London, focused her dissertation on the optimisation of InAs quantum dots to maximise the open circuit voltage of the solar cells while keeping extended absorption. Finally, Kasidit Toprasertpong, from the University of Tokyo, presented the latest results on multi-quantum wells and superlattices aiming to 1.15 eV absorption edge for lattice-matched 4-junction solar cells. A final theoretical talk was given by Urs Aeberhard who showed the limits of the classical, bulk-like description of photovoltaics – in terms of absorption and transport properties – in the frontier between the macro and the nano worlds, for example in the presence of very thin absorbers but before reaching the quantum size.

In summary, the workshop was a complete success, especially in terms of the quality of the speakers, coming from 8 different countries and 3 continents, and the fruitful discussions in the breaks, serving to both strengthen existing collaborations or laying out plans for future projects. The number of posters (10) was also the largest of all the editions of HEMP, so far, encouraging informal talks and networking.